Particular types of electro-optical modulators have been recently developed, such as, for example, described in Electronic Letters 9, 1973 pages 309 and 310 and in Proceedings of IEE Vol 19 + No. 7, 1972 pages 807 to 814. The operation of these modulators depends on the effect of applying a voltage to a symmetrical electrode pattern to induce periodic change of the refractive index in an electro-optical element in the region of the surface of the element at the moment of total internal reflection. The resulting symmetrical periodic phase change induced in the wave-front of a light beam directed through or at the electro-optical element produces a far field pattern of the Raman-Nath form exhibiting symmetry about the zero energy order. The zero order of the output beam can then be modulated by adjusting the applied voltage to the electrode pattern to alter the distribution of the light from the zero order into higher orders, and as often desired, to eliminate the zero order.
The electro-optical modulators so far described have operated on light beams of single wavelengths. It would be desirable if such a modulator can be provided which operate on light beams of more than wavelength.